1. Field of the Invention
This invention relates to a curing agent and a coating composition, more specifically to a curing agent containing an isocyanurate ring and a cationic electrodeposition coating composition which contains it.
2. The Prior Art
A curing agent normally used in a cationic electrodeposition paint is a multifunctional blocked isocyanate compound. This type of multifunctional blocked isocyanate compound is generally obtained by reacting a half-blocked diisocyanate compound with a polyhydric alcohol compound.
When a cationic electrodeposition paint which contains such a curing agent is placed in an overbaking condition at 200.degree. C. or higher, the urethane bonding of the curing agent redissociates and the obtained diisocyanate compound is apt to volatilize, leading to an increase in heating loss. Such heating loss is believed to be a possible cause of a decrease in film thickness and coloring on the topcoating film.
A multifunctional blocked isocyanate compound containing an isocyanurate ring which is a trimer of isocyanate has been considered as a curing agent which does not easily cause heating loss.
For example, Japanese unexamined patent publication Tokkai Hei 1-266172 and Tokkai Hei 2-11669 disclosed a curing agent comprising a multifunctional blocked isocyanate compound containing an isocyanurate ring from a trimer of hexamethylenediisocyanate, which is an aliphatic diisocyanate. However, although this curing agent has the effect of reducing the heating loss, corrosion resistance of the coating film may be reduced.
Japanese examined patent publication Tokko Sho 52-6306, Tokko Sho 55-34238 and Tokkai Hei 4-91170 disclosed use of a multifunctional blocked isocyanate compound containing an aromatic isocyanurate ring as a curing agent. However, although this curing agent also can be expected to have the effect of reducing the heating loss, it often reduces the flatness of the electrodeposition coating film.